Tractor-type drive for boats with trim plane

ABSTRACT

A drive for a boat includes a gear case and a propeller rotatably mounted to a front of the gear case. The drive also includes a drive support for mounting the gear case to the boat. The drive support allows at least a portion of the gear case to pivot about a trim axis to adjust the boat&#39;s trim. A trim plane is formed as part of, or added to, the gear case between a lower portion of the gear case, usually underwater in use, and an upper portion of the gear case. The trim plane has sufficient exposed lower surface area to apply a downward force on the water to assist trimming of the boat.

TECHNICAL FIELD

This disclosure relates to marine drives. Particularly, this disclosure relates to tractor-type drives having forward facing propellers configured to pull a boat through the water.

BACKGROUND

Marine drives may be generally classified as inboard, outboard, or inboard/outboard. In an inboard drive, the engine and transmission are mounted in the hull and a propeller shaft extends through the bottom of the hull. In an outboard drive, the propeller drive and engine are generally configured as a unit attached to and located outside the hull. Inboard/outboard drives, also referred to as stern drives, have an engine mounted in the hull connected to a drive unit mounted outside of the hull, typically on the stern.

Marine drive units can be further classified as pushing-type and tractor-type. Pushing-type drives generally rely upon propellers facing rearward relative to the boat and generating propulsive force that pushes the boat through the water. Tractor-type drives generally rely upon one or more forward, bow-facing propellers that produce propulsive force to pull the boat through the water. Tractor-type drives may also be referred to as pulling-type drives.

Whether using a pushing-type or a tractor-type drive, the rearward weight distribution of stern drives and outboards can cause the stern of the boat to dip down into the water as the bow rises above the water at high speeds. This dipping of the stern increases drag and can leave a large wake trailing behind the boat. Efficiency and handling are optimized at high speed if the hull is able to stay “on plane” and sit as high and flat in the water as possible.

To assist the boat's ability to be on plane, several mechanisms exist for trimming or adjusting the trim angle of the boat. Trimming the boat allows the operator to adjust the front-to-back or back-to-front tilt of the boat. Some outboard and sterndrive boats adjust the trim by adjusting the relative angle between the hull of the boat and the shaft rotating the propeller, i.e. the “propeller shaft” as used herein. A drive trim angle of zero degrees generally aligns the propeller shaft parallel with the longitudinal axis of the boat. Trimming in, also called trimming down, creates a negative drive trim angle that brings the propeller closer to the stern and tends to push the stern of the boat up out of the water, bringing the bow down into the water. Trimming out, also called trimming up, creates a positive drive trim angle that pivots the propeller away from the boat and tends to force the stern of the boat down into the water, raising the bow up out of the water.

FIG. 1 shows a sequence for trimming a boat having a conventional pushing-type sterndrive. FIG. 1A shows the boat with a zero trim angle while the boat is still. The longitudinal axis L of the boat is parallel with the surface of the water W when the boat and water are still. The longitudinal axis L is parallel with the propeller shaft axis P and perpendicular to a drive shaft axis Z when the drive is at no or zero trim angle (indicated as “Z₀”). As seen in FIG. 1B, acceleration of the boat in the forward travel direction causes the bow to raise and the stern to dip into the water, in part because of the rearward center of gravity. To reach top speed, the drive unit 1 is then trimmed up (or out) as shown in FIG. 1C to reduce the drag of the bow through the water. With respect to the drive shaft axis Z₀ relative to the longitudinal axis L at zero trim, trimming up pivots the top of the drive unit 1 toward the stern of the boat and pivots the bottom of the drive unit 1 away from the stern of the boat (indicated by drive shaft axis Z_(up) at positive trim angle). At cruising speed shown in FIG. 1D, the drive unit 1 may be trimmed in (or down) for stability as the moves across the water. With respect to the drive shaft axis Z relative to the longitudinal axis L, trimming in pivots the top of the drive unit 1 away from the stern of the boat and pivots the bottom of the drive unit 1 toward the stern of the boat (indicated by drive shaft axis Z_(down) at negative trim angle).

SUMMARY

An embodiment of this disclosure includes a tractor-type drive for a boat. The drive includes a gear case and a drive support for mounting the gear case to the boat. The drive support allows at least a portion of the gear case to pivot about a trim axis. A pulling-type propeller is rotatably mounted to a front of the gear case. The gear case has a trim plane defining a boundary between a lower, underwater portion and an upper, above-water portion of the gear case. The trim plane has sufficient surface area to apply a downward force on the water to assist trimming of the boat.

An embodiment of this disclosure also includes a drive for a boat. The drive has a gear case and a drive support for mounting the gear case to the boat. The drive support allows at least a portion of the gear case to pivot about a trim axis. A propeller carried on a shaft is rotatably mounted at a front of the gear case. The gear case includes a trim plane defining a boundary between an underwater portion and an above-water portion of the gear case. The trim plane has sufficient size to apply a downward force on the water to assist trimming of the boat.

An embodiment of this disclosure also includes a boat. The boat has a hull and a drive attached to a stern of the hull. The drive includes a gear case, at least one propeller rotatably mounted to a forward end of the gear case, and a drive support. The drive support is used to mount the gear case to the stern of the boat, and allows the gear case to pivot relative to the boat about a trim axis. The gear case includes a trim plane defining a boundary between an underwater portion and an above-water portion of the gear case. The trim plane has a lower surface with an exposed surface area of at least about 70,000 mm² to apply a sufficient downward force on the water to assist trimming of the boat.

These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiments, when considered in conjunction with the drawings. It should be understood that both the foregoing general description and the following detailed description are explanatory only and are not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawings in which like numerals indicated similar elements and in which:

FIGS. 1A-D illustrate a prior art description of trimming a boat having a stern drive;

FIG. 2 shows a tractor-type drive with a trim plane according to embodiments of the present disclosure; and

FIG. 3 shows the tractor-type drive of FIG. 2 with the upper gear case omitted.

DETAILED DESCRIPTION

Exemplary embodiments of this disclosure are described below and illustrated in the accompanying figures, in which like numerals refer to like parts throughout the several views. The embodiments described provide examples and should not be interpreted as limiting the scope of the invention. Other embodiments, and modifications and improvements of the described embodiments, will occur to those skilled in the art and all such other embodiments, modifications and improvements are within the scope of the present invention. Features from one embodiment or aspect may be combined with features from any other embodiment or aspect in any appropriate combination. For example, any individual or collective features of method aspects or embodiments may be applied to apparatus, product or component aspects or embodiments and vice versa.

As used herein, the terms “front” and “forward” are defined based on the drives as mounted to the boat with respect to a bow to stern direction of the boat. Likewise, the terms “back”, “rear”, “rearward”, and “aft” are also defined based on the drive as mounted to the boat with respect to a bow-stern direction of the boat.

This disclosure relates to outboards and sterndrives with drives 100 of the tractor-type. Applicants have determined that tractor-type drives can have reduced trim response in comparison to similar pushing-type sterndrives. Therefore, there is a need for a tractor-type drive with improved trim response.

Tractor-type drives were expected to improve trimming function relative to pushing-type drives. When in a trimmed out position, the propellers in a pushing-type drive are positioned in aerated, less dense water and become less effective. To the contrary, when trimmed out, pulling propellers of the tractor-type drive are positioned deeper in the water, which is less likely to result in the propellers ventilating air. However, testing did not show the expected results.

One possible explanation for the greater trim response of pushing-type sterndrives could be the added trimming forces provided by anti-ventilation plates commonly located above the propellers in pushing-type drives. A conventional anti-ventilation plate has high pressure underneath it from the propellers. The force from the high pressure, together with the drive trim angle, lifts the stern of the boat when trimmed in. However, tractor-type drives are less likely to suffer from ventilation issues because their propellers are further under the boat, and anti-cavitation plates are regularly omitted.

FIG. 2 shows a boat 200 (in section view) with a tractor-type drive 100. The drive 100 is configured to be mounted to the stern 210 of the boat 200. The drive 100 includes at least one pulling (or tractor) propeller, which can be configured as a forward propeller 104 and a rearward propeller 108, mounted to a front end of a gear case 120. The forward and rearward propellers 104, 108 in the illustrated embodiment are a pair of counter-rotating propellers mounted on coaxially rotating shafts.

A tractor-type drive 100 has been shown to have several benefits over the more common pushing-type drives, which benefits include improved range, higher speeds, reduced fuel consumption and fewer emissions. Particularly, forward facing propellers are positioned in less disturbed water, which increases the ability of the propellers to impart energy to the water and propel the boat. Placing the propellers further under the boat allows them to be more likely to remain submerged when the drive is trimmed, allowing for higher trim angles at slow speeds. Moving the propellers forward reduces the exposure of swimmers at the rear of the boat. Use of the tractor-type drives also allows the exhaust to be directed into the propeller wash, conveying the exhaust further rearward of the boat.

The drive 100 can also include a drive support 140 for mounting the gear case 120 to the stern 210 of the boat 200, in particular, to the transom 220. In some embodiments, the drive support 140 mounts the gear case 120 to the transom 220 such that the forward and rearward propellers 104, 108 are disposed substantially rearward of the transom 220. The drive support 140 allows the gear case 120 to pivot relative to the boat 200. In the illustrated embodiment of FIG. 2, the drive support 140 includes a transom shield 142 fixed to the transom 220 and a gimbal ring 144 pivotably mounted to the transom shield. The gimbal ring 144 is pivotal about a substantially vertical steering axis S. The gear case 120 is mounted to the gimbal ring 144 for pivoting about a substantially horizontal tilt/trim axis T. The gear case 120 can thus pivot relative to the boat 200 about at least the steering axis S and the tilt/trim axis T.

By pivoting the gear case 120 about the steering axis S, the drive 100 is able to steer the boat 200 by directing the thrust of the propellers. In addition, the underwater portion 124 of the gear case 120 acts as a rudder. By pivoting the gear case 120 about the trim axis T, the drive 100 is able to change the trim angle of the boat 200. In the illustrated embodiment, at least one piston 122 extends when trimming out the gear case 120 and the at least one piston 122 contracts to trim in the gear case 120.

The gear case 120 includes the lower portion 124 generally positioned below the surface of the water W when the gear case 120 is in use (mounted on a boat in the water), and an upper portion 128 generally positioned above the surface of the water W when the gear case 120 is in use. According to the invention, a trim plane 130 is positioned between the lower portion 124 and the upper portion 128. The trim plane 130 may be integrally formed with the gear case 120. In other embodiments, the trim plane 130 may be separately attachable and removable from the gear case 120. Whether integral or removably attached, the trim plane 130 is expected to move with the gear case 120 during pivoting about the steering axis S. The trim plane 130 may also be referred to in this disclosure or in the industry as a trim plate.

Referring now also to FIG. 3, the trim plane 130, being between the lower portion 124 and the upper portion 128, should be understood to be operatively positioned substantially at the surface of the water W such that a lower surface 133 of the trim plane 130 contacts the water and the upper surface 136 may be above or slightly below the surface of the water W. The location of the trim plane 130 is important to its function assisting with the trim of the boat 200. The trim plane 130, particularly the lower surface 133, is designed to apply a downward force on the water, thereby assisting to force the stern 210 of the boat 200 up out of the water. The downward force provided by the trim plane 130 increases as the drive 100 is trimmed in, pressing the aft end 139 of the trim plane 130 down toward, or into, the water. In some embodiments, the lower surface 133 is planar.

The amount of trimming assistance provided by the trim plane 130 is proportional to the area of the trim plane 130, particularly the surface area of the lower surface 133 that is exposed to the water. The exposed surface area of the lower surface 133 should be at least about 70,000 mm². In one embodiment, as used in the tests reported below, the exposed surface area of the lower surface 133 is about 90,000 mm². Having a surface area less than about 70,000 mm² may provide insufficient force on the water to sufficiently improve trim function.

In some embodiments, the trim plane 130 is symmetric about a centerline of the gear case 120. Symmetry adds to the balance and stability of the drive 100. In other embodiments, for example, in paired drives mounted adjacent on a boat, the trim plane 130 may be asymmetric about the centerline.

The trim plane 130 may be designed to position a large proportion of the surface area as far in the aft direction as possible. By positioning the surface area as far rearward as possible, the moment arm of the trim force is increased, further assisting the trimming of the boat 200. As seen in FIG. 3, the trim plane 130 has a first width W_(F) across a relative front portion of the trim plane 130, and a second width W_(A) across relative aft section of the trim plane 130 such that W_(A)≧W_(F). In some embodiments, the first width W_(F) is less than the second width W_(A) such that the trim plane 130 tapers from back to front. Further, the second width W_(A) may be the maximum width.

In some embodiments, in a fore-aft direction, the trim plane's front end 142 is disposed substantially rearward of the forward and rearward propellers 104, 108. In other words, the trim plane 130 is not disposed above the forward and rearward propellers 104, 108 and is not expected to provide an anti-ventilation function. The trim plane's front end 142 is also disposed rearward of the stern 210 of the boat 200.

Testing

The trim performance of a boat powered by the tractor-type drive with a trim plane, according to a preferred embodiment of the present disclosure, was compared to the trim performance of the same boat powered by a substantially similar pushing-type drive (the Volvo Penta DPS-B) and a substantially similar tractor-type drive without the trim plane feature. The tested embodiment of the trim plane had an exposed area of 90,000 mm² and a shape as shown in FIGS. 2 and 3. The three drives were run at drive trim angles of −5, 0 and 5 degrees over a range of engine speeds where the boat pitch angle was recorded. The results are shown in Table 1 below.

TABLE 1 Boat Pitch Angle vs. RPM Drive Boat pitch angle in degrees Trim (°) Engine RPM 2000 2500 3000 3500 4000 4500 −5 Pushing-type Drive 6 2.3 1.17 0 −0.8 −1 Tractor-Type Drive 5.7 2.9 2.3 2.1 1.5 1.0 Tractor-Type Drive 6.1 2.0 0.0 −0.2 −0.9 −1 with Trim Plane 0 Pushing-type Drive 7.0 3.25 1.3 0.45 0 −0.4 Tractor-Type Drive 8.5 6.2 4.7 4.0 2.7 1.6 Tractor-Type Drive 7.1 3.2 0.8 −0.2 −0.8 0.3 with Trim Plane +5 Pushing-type Drive 9.5 4.9 2.9 0.1 0.3 −0.9 Tractor-Type Drive 9.6 7.1 5.3 4.3 2.2 1.3 Tractor-Type Drive 9.6 4.9 2.0 0.1 −0.6 −1.0 with Trim Plane

As seen from the results shown in Table 1, the boat pitch experienced by the boat powered with the trim plane equipped tractor-type drive improved over a tractor-type drive without a trim plane (or with a small trim plane) and much more closely mirrored the boat pitch experienced by the boat powered by the pushing-type drive.

Although the above disclosure has been presented in the context of exemplary embodiments, it is to be understood that modifications and variations may be utilized without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims and their equivalents. Features from one embodiment or aspect may be combined with features from any other embodiment or aspect in any appropriate combination. For example, any individual or collective features of method aspects or embodiments may be applied to apparatus, product or component aspects or embodiments and vice versa. 

We claim:
 1. A tractor-type drive for a boat, comprising: a gear case; a drive support for mounting the gear case to the boat, the drive support configured to allow at least a portion of the gear case to pivot about a trim axis; a pulling-type propeller mounted on a propeller shaft extending forward from a front of the gear case; wherein the gear case comprises a trim plane between a lower portion and an upper portion of the gear case, the trim plane configured to apply a downward force on the water to assist trimming of the boat.
 2. The drive of claim 1, wherein the trim plane has an upper surface and a lower surface, wherein the lower surface has a surface area of at least about 70,000 mm² exposed to the water.
 3. The drive of claim 2, wherein the lower surface has a surface area of about 90,000 mm² exposed to the water.
 4. The drive of claim 1, wherein the trim plane has a first width at a forward end thereof and a second width at an aft end thereof, and wherein the first width is less than or equal to the second width.
 5. The drive of claim 4, wherein the first width is less than the second width such that the trim plane tapers from back to front.
 6. The drive of claim 1, wherein the drive support is configured to allow at least a portion of the gear case to pivot about a steering axis to steer the boat, wherein the trim plane pivots with the gear case about the steering axis.
 7. The drive of claim 1, wherein the trim plane has a front end, the front end of the trim plane begins rearward of the pulling-type propeller.
 8. The drive of claim 1, wherein, when mounted on a boat, the drive support is configured to position the pulling-type propeller substantially rearward of the boat's transom.
 9. The drive of claim 1, wherein the trim plane has an upper surface configured to be above the water and a lower surface configured to be at or below the water, the lower surface being planar.
 10. A boat, comprising: a hull; and a drive attached to a stern of the hull, the drive comprising: a gear case; at least one propeller rotatably mounted at a forward end of the gear case; and a drive support mounting the gear case to the stern of the hull such that the gear case is able to pivot about a trim axis, wherein the gear case comprises a lower portion and an upper portion and a trim plane between the lower portion and the upper portion, the trim plane has a lower surface with an exposed surface area of at least about 70,000 mm² to apply a downward force on the water to assist trimming of the boat.
 11. The drive of claim 10, wherein the lower surface has a surface area of about 90,000 mm² exposed to the water.
 12. The drive of claim 10, wherein the trim plane has a first width at a forward end thereof and a second width at an aft end thereof, wherein the first width is less than or equal to the second width.
 13. The drive of claim 12, wherein the first width is less than the second width such that the trim plane tapers from back to front.
 14. The drive of claim 10, wherein the drive support is configured to allow at least a portion of the gear case to pivot about a steering axis to steer the boat, wherein the trim plane pivots with the gear case about the steering axis.
 15. The drive of claim 10, wherein the trim plane has a front end, the front end of the trim plane is positioned rearward of the at least one propeller.
 16. The drive of claim 10, wherein, the drive support is configured to position the at least one propeller substantially rearward of the transom.
 17. The drive of claim 10, wherein the trim plane has an upper surface configured to be above the water and a lower surface configured to be at or below the water, the lower surface being planar. 